ReviewBiomechanics of the optic nerve head
Section snippets
Ocular biomechanics in glaucoma
Elevated intraocular pressure (IOP) remains the primary risk factor for development of glaucomatous optic neuropathy (Lesk et al., 2003, Bengtsson and Heijl, 2005, Heijl et al., 2002), and consistent, sustained and significant reduction of IOP slows or eliminates visual field loss in glaucuma (AGISInvestigators, 2000, Anderson et al., 2001, Heijl et al., 2002, Lesk et al., 2003). IOP is, by definition, a mechanical entity – the normal force per unit area exerted by the intraocular fluids on the
Cellular mechanobiology
Cells are sensitive to many stimuli, including mechanical stimuli. Before describing some of the evidence supporting mechanical factors as important influences on cellular behavior, it is worth introducing some terms from biomechanics.
Strain is the change in length of a tissue element divided by its initial length (Humphrey, 2002, Ethier and Simmons, 2007), and is thus a measure of the local tissue deformation, usually expressed as a percentage. As it deforms, a material can undergo tension,
Quantifying lamina cribrosa biomechanics
Based on the above, as well as the possibility that mechanical forces may lead to direct failure (tearing) of connective tissue fibers in the ONH (Burgoyne et al., 2005), it seems important to understand the biomechanical environment within the LC. Unfortunately, it is difficult to make measurements on the LC directly because it is small, fragile and relatively inaccessible.
Some researchers have studied the movement of the vitreoretinal surface of the ONH as a surrogate for LC motion (Zeimer
The influence of the sclera
A somewhat surprising outcome of the above models is the suggestion that the properties of the sclera have a stronger-than-expected influence on the biomechanics of the ONH. Soft tissues, such as the sclera, often exhibit complex mechanical responses to loading, which may be necessary for the proper description of the effects of IOP on the ONH (Fung, 1993). This motivates studies designed to characterize the biomechanical properties of the sclera, particularly of the peripapillary sclera. The
The influence of retrolaminar tissue pressure
When considering LC biomechanics it is natural to focus on the role of IOP, since it is clinically observable and clearly important. However, from a biomechanical viewpoint, we should remind ourselves that it is the difference between IOP and retrolaminar tissue pressure (RLTp) that directly loads the LC. Put another way, if this pressure difference were zero, then the LC would experience zero net direct loading. Note however this does not mean that the lamina would be free of all biomechanical
Conclusions
The biomechanics of the LC are complex and potentially important in glaucomatous optic neuropathy. The stiffness and thickness of the peripapillary sclera and the microarchitecture of the laminar beams both have a strong effect in the mechanical insult experienced by cells in the LC. Studies in which numerical modeling is informed by experiments offer a powerful way to better understand LC biomechanics. Such characterization of the biomechanical environment within the LC must in turn be coupled
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